Surface treatment of co-cr based alloys using plasma carburization

ABSTRACT

The present invention relates to a method of modifying a surface characteristic (e.g., wear resistance and/or corrosion resistance) of a cobalt-chromium based alloy article. The method comprises plasma treating the article at a temperature in the range of from 300° C. to 700° C. and at a pressure of from 100 Pa to 1500 Pa for 1 hour to 50 hours in an atmosphere comprising at least one carbon-containing gas, whereby to introduce carbon into a surface region of said article. The present invention also resides in a surface-hardened cobalt-chromium based article producible by the method of the invention. The article is characterised by having a surface region comprising a supersaturated solid solution of carbon in cobalt or a surface region comprising a supersaturated solid solution of carbon in cobalt and chromium carbides. Surface hardened articles producible by the method of the invention include medical implants and engineering components.

The present invention relates to a process for producing a superiorwear-resistant surface on cobalt-chromium (Co—Cr) based alloy articles.Particularly but not exclusively, it relates to a surface hardeningprocess applicable to cobalt-chromium (Co—Cr) based metal-on-metalorthopedic implant devices (prostheses), wherein surface hardness andwear resistant properties of the prostheses are enhanced without loss ofcorrosion resistance.

Cobalt-chromium based alloys have typically been used for orthopedicapplications because of their strength, resistance to wear and corrosionand biocompatibility. However, under conditions of sliding wear orarticulation of the Co—Cr alloy against other bearing surfaces(particularly, ceramics or Co—Cr alloy counterfaces), thecobalt-chromium alloy produces wear debris from the counterface surfacesin relative motion. This raises a major concern over the carcinogeniceffect of such Co—Cr wear debris and the release of such metal ions asCo and Cr. Therefore, the surface of the Co—Cr alloys must be hardenedin order to minimise wear, thus leading to long-life Co—Cr orthopedicprostheses. Several methods for improving wear resistance of Co—Cr basedalloys and such articles made from these alloys have been attempted.

One approach to enhancing the wear performance of metallic femoralcomponents (hip and knee prostheses) is to coat their surfaces with suchceramic coatings as TiN (J. A. Davidson, Ceramics in substitutive andreconstructive surgery (P. Vincenzini ed), Elsevier, Amsterdam, 1991 pp157-166) and more recently diamond-like carbon (DLC) coating (M. Allen,J. Biomed, Mater. Res., 58 (2001), pp 319-328). Potential drawbacks tothis approach are concerns over spallation of coatings (M. T. Raimondiand R. Pietrabissa, Biomaterials, 21 (2000), pp 907-913) due to thenon-metallurgical bonding and galvanic corrosion (caused by the largegalvanic potential difference and pinholes in coatings) (R. S. Lillardet. al., Surface Engineering 15 (1999), pp 221-224). Although a TiNcoating has been explored for use as a bearing surface against UHMWPEsince the late 1980's, laboratory testing and clinical results have beenlimited. Indeed, it was reported in 1998 that 5,000 hip operations mighthave to be repeated in the UK, the problem being attributed to theTiN-coated surface (R. Ellis et, al., The Times, 19 Feb. 1998, p1.).Although simple configuration laboratory tests indicated that the DLCcoatings had exceptional friction and wear characteristics, tests on hipsimulators produced early failures of the coatings (A. H. S. Jones andD. Teer ‘Friction and wear testing of DLC type coatings on total hipreplacement prostheses’ Seminar on The Friction Lubrication and Wear ofArtificial Joints—Tribology Meets Medical, Institute of MechanicalEngineers, 30 Nov. 2000, Leeds). Clearly there is cause for concern inusing coatings for metallic femoral components. The problem is mainlydue to the formation of an oxide scale (Cr₂O₃) on the alloy surface dueto the strong affinity of chromium, which is a major alloying element ofCr—Co alloys, with oxygen in air. This oxide scale frequently cause apoor adhesion between a coating and the Co—Cr alloy surface. Surfacetreatment of Co—Cr alloys usually has to overcome this major problem,and consequently, such coating techniques as PVD coating,electroplating, and electrolysis plating have limitations for Co—Cralloys, as compared with coating and plating for most ferrous alloys.

Another approach to improving the wear performance of femoral componentsis to modify the metallic surface. In this respect, ion implantationwith nitrogen has been employed since the mid 1980's to improve wearresistance of metallic bearing surfaces made of TI-6Al-4V, 316L andCo—Cr—Mo (J. I. Onate, Surface and Coatings Technology, 142-144 (2001),pp 1056-1062). However, it should be noted that the effectiveness of ionimplantation in enhancing wear resistance of Co—Cr—Mo alloys formetal-on-metal articulation is limited by the inherent line-of-sightnature of the ion beam and the very thin modified layer which isproduced. It is difficult, if not impossible, to produce a homogeneoussurface modified layer on 3-D complexly shaped prostheses using theline-of-sight ion beams. In addition, the thickness of the modifiedsurface layer (normally in the range of 0.01 to 0.2 μm) is far less thanthe average annual linear wear rate of Co—Cr—Mo prostheses.

In addition to ion implantation with nitrogen, known surface-hardeningmethods include gas nitriding or plasma nitriding. A nitriding processis disclosed in detail in U.S. Pat. No. 5,912,323 issued on May 3, 1994entitled “METHOD OF SURFACE HARDENING COBALT-CHROMIUM ALLOYS FORORTHOPAEDIC IMPLANT DEVICES”, which disclosure is hereby incorporated byreference. To produce a measurable hardened layer, a treatment durationas long as 48 hours needs to be used. The thickness of the effectivelyhardened layer is very small as evidenced by the statement that “thepeak nitrogen concentration occurs at a depth between 10 and 100 nm”.This is largely due to the strong affinity between nitrogen andchromium. Indeed, it was found that after plasma nitriding at 550° C.for 8 h in a 75% N₂-25% H₂ gas mixture, only a compound layer of CrN wasobserved in the nitrided case of Stellite 6B, a Co-30Cr alloy withoutthe formation of an appreciable diffusion zone (P. H. Howill, Ionnitriding stellite in T. Spaluins and W. I. Kovaes (eds): Proceedings of2^(nd) International Conference of Ion Nitriding/Carburising. ASMInternational 1990, 175-176).

It is an object of the present invention to provide an improved methodof treatment of Co—Cr based alloy articles, which can enable theabove-mentioned disadvantages to be obviated or mitigated.

In general, the present invention provides an improved surface hardeningprocess which is relatively cost-effective and which is capable ofproducing, at a relatively low temperature, combined improvement in wearand corrosion resistance of Co—Cr based alloy joint prostheses (such aship and knee joints) and producing highly wear-resistant Co—Cr basedalloy engineering components (such as valves and tools) without undueloss in corrosion resistance.

According to the present invention, there is provided a method ofmodifying a surface characteristic of a cobalt-chromium based alloyarticle, comprising plasma treating the article at a temperature in therange of from 300 to 700° C. and at a pressure of from 100 to 1500 Pafor 1 to 50 hours in an atmosphere comprising at least onecarbon-containing gas, whereby to introduce carbon into a surface regionof said article.

The surface characteristic to be modified by the method of the presentinvention may be any one or more of hardness, wear resistance, corrosionresistance and fatigue strength.

Preferably, said article is a medical implant, such as a joint or kneeprosthesis, in which case said plasma treating is preferably carried outat a temperature in the range of from 350 to 550° C., and morepreferably 400 to 500° C. At these temperatures, the method generallyincreases wear resistance and corrosion resistance.

Alternatively, said article may be an engineering component, such as aknife, valve, blades or shaft, in which case said plasma treating ispreferably carried out at a temperature in the range of from 450 to 700°C., more preferably 600 to 650° C. At these temperatures, the methodgenerally increase wear resistance, but not necessarily corrosionresistance.

Preferably, said treatment pressure is in the range of from 400 to 600Pa and is more preferably about 500 Pa.

Preferably, the duration of said treatment is in the range of from 2 to50 hours and more preferably 5 to 30 hours.

Preferably, the or each carbon-containing gas is selected from ahydrocarbon (eg, methane), carbon dioxide and carbon monoxide.

Preferably, the plasma treatment is carried out in the presence of atleast one unreactive gas, for example selected from hydrogen, helium,argon or other noble gas. As used herein “unreactive” relates to a gaswhich does not become incorporated into the article to any significantextent.

Preferably, the plasma treatment is carried out in the presence of atleast one reactive gas, such as a nitrogen containing gas (eg. N₂ orammonia). As used herein “reactive” relates to a gas which (or a part ofwhich) does become incorporated into the article to a certain extent.Where a nitrogen-containing gas is used, the plasma treating step ispreferably effected at a temperature of from 300 to 500° C.

Particularly preferred gas mixtures are hydrogen and methane, andhydrogen, argon and methane.

Preferably, the or each carbon-containing gas constitutes from 0.5 to20% by volume of the total atmosphere. Preferably, said reactive gas(when present) constitutes from 0.5 to 10% by volume of the totalatmosphere.

Preferably, said plasma treatment is effected in the absence of oxygen.

The method may include an article cleaning step prior to the plasmatreatment step to remove any oxide scale. Preferably, said cleaning iseffected by sputter cleaning (i.e. bombardment of the article surfacewith positive ions). Said cleaning step may be effected at or below theplasma treatment temperature in an atmosphere of one or more of gasesselected from hydrogen, helium, argon or other noble gas.

It will be understood that after plasma treating, the article will becooled. The rate of cooling may be anything from 0.1° C./min up to 1000°C./min. Cooling may be achieved by slow cooling in the plasma treatingatmosphere or by fast cooling by quenching in a liquid. In order toprevent dimension distortion and oxidation, slow cooling in the plasmatreating atmosphere is preferred.

Particularly for medical implants, a passivation and/or polishing stepmay be desirable after completion of the plasma treatment.

The present invention also resides in a surface-hardened cobalt-chromiumbased article producible by the method of the present invention, saidarticle characterised by having:—

(i) a surface region comprising a supersaturated solid solution ofcarbon in cobalt or,

(ii) a surface region comprising a supersaturated solid solution ofcarbon in cobalt and chromium carbides,

Preferably, said surface region has a thickness in the range of from 3to 50 μm.

The nature of the Co—Cr based alloy is not particularly limited, and forexample, any other alloying ingredients such as molybdenum, nickel,tungsten and titanium may be included in the alloy composition. Carbonmay also be included; preferably in the range of from 0.04 to 1.6 wt %,more preferably in the range of 0.04 to 0.4 wt % in the case of amedical implant, and 0.4 to 1.6 wt % in the case of an engineeringcomponent.

Among the Co—Cr based alloys which are useful for joint prostheses areASTM F75 (1505832/4), ASTM F799 (1505832/12), ASTM F90 (ISO5832/5) andASTM F562 (ISO5832/6) or their equivalents with different trade names.Articles formed of these alloys which can be surface treated at arelatively low temperature (300-500° C.) in accordance with the presentinvention include conventional hip and knee joint prostheses,metal-on-metal advanced bone conservation prostheses, dental implantsand other implant devices. Among the Co—Cr based alloys which are usefulfor wear and/or corrosion resistant engineering components are Stellite6B, Stellite 6K, MP35N and Ultimet. Wear-resistant engineeringcomponents made of these Co—Cr based alloys which can be surface treatedat a relatively high temperature (600-700° C.) in accordance with thepresent invention include knifes and blades for chemical and foodprocessing industrials, valves and pumps in the chemical and powerindustrials, bushings and steel mill equipment. Among the Co—Cr basedalloys which are useful for hardfacing deposits are the Stellite family(more than 20 alloys) and ERCoCr alloys (ERCoCr-A, -B, -C or -E).Articles deposited with these hardfacing alloys which are preferablysurface treated at a relatively high temperature (300-700°C.) inaccordance with the present invention include valves, dies, punches,moulds, turbine blades and knifes.

Embodiments of the present invention will now be described by way ofexample only, with reference to the accompanying drawings in which:—

FIG. 1 is a schematic view of a dc plasma unit in which the treatmentdescribed in the preferred embodiments below was effected,

FIG. 2 is a micrograph of the cross-sectional microstructure of aCo—Cr—Mo test piece treated in accordance with the present invention,

FIG. 3 shows XRD patterns for untreated test pieces and test piecessurface hardened in accordance with the present invention, and

FIGS. 4-6 are graphs showing the properties of untreated test pieces andtest pieces surface hardened in accordance with the present invention.

Typical examples of suitable Co—Cr based alloys which are susceptible tothe process of the present invention are summarised in Table 1. TheCo—Cr based alloys of which the article is formed may be in the wrought,cast, hardfacing deposit or PM/HIP form before the article is subjectedto the process to the present invention.

TABLE 1 Examples of useful Co—Cr based alloys Alloy Cr Mo Ni W C Ti CoWear-corrosion resistant biomedical alloys F75 (ISO5832/4) 27-30 5-7 <1— <0.35 — bal F799 (ISO5832/12) 26-30 5-7 <1 — <0.35 — bal F90(ISO5832/5) 19-21 —  9-11 14-16 <0.15 — bal F562 (ISO5832/6) 19-21  9-1133-37 — <0.15 <1 bal Wear-resistant alloys Alloy 6B 28-32 <1.5 <2.5 3-50.8-1.2 — bal Alloy 6K 28-32 <1.5 <2.5 3.5-5.5 1.5-1.7 — balCorrosion-resistant alloys MP35N 19-21  9-11 34-36 — — — bal Ultimet25-27 4-6  8-10 1-3 <0.06 — bal Hardfacing alloys ERCoCr-A ~28 — — ~5~1.2 — bal ERCoCr-B ~29 — — ~8 ~1.5 — bal ERCoCr-C ~31 — — ~13  ~2.5 —bal ERCoCr-E ~27 ~6 — ~8 ~0.2 — bal

The surface-treatment-process can be applied as a final procedurewithout causing deterioration of the properties of the substrate ordimensional distortion of the article. Articles for which the process ofthe present invention is suitable include such articles as ferrules,valves, gears and shafts. There is no particular limit in the size ofarticles that can be treated using the process of the present invention.

In order to demonstrate the advantages of the present invention, aseries of Co—Cr based alloys (Table 2) were treated in accordance withthe present invention.

In the Examples, surface treatment was carried out using a dc plasmanitriding apparatus shown in FIG. 1. The apparatus comprises a sealablevessel 10, a vacuum system 12 with a rotary pump (not shown), a dc powersupply and control unit 14, a gas supply system 16, a temperaturemeasurement and control system 18 including a thermocouple 24, and awork table 20 for supporting articles 22 to be treated.

TABLE 2 Alloy composition of the Examples Sample designation* ConditionComposition (% by wt) A Wrought Co—27.6Cr—5.5Mo—0.06C B WroughtCo—37.4Cr—6.1Mo—0.19C C Cast (MMT) Co—29.2Cr—6.1Mo—0.21C D Cast (DEP)Co—27.2Cr—0.17C E Cast (DRILL) Co—29.6Cr—5.9Mo—0.05C F PM/HIPCo—28.2Cr—5.8Mo—0.04C *For each sample designation, a different samplewas treated at 400, 500 and 600° C. (designated A400, A500, A600 etc.)

The articles 22 to be treated were Co—Cr based alloy discs 25 mm indiameter and 8 mm in thickness. The discs were placed on the table 20inside the vessel 10. The table 20 was connected as a cathode to thepower supply and control unit 14, and the wall of the vessel 10 wasconnected to the dc source as the anode. The temperature of the discs 22was measured by the thermocouple 24 inserted into a hole of 3 mmdiameter drilled in one of the discs 22 or a dummy sample. After thesealable vessel 10 was tightly closed, the rotary pump was used toremove the residual air (oxygen) and thus reduce the pressure in thevessel. When the reduction in pressure reached 10, Pa (0.1 mbar) orless, a glow discharge was introduced between the article 22 (cathode)and the vessel wall (anode) by applying a voltage of 400 volts to 900volts between these two electrodes. A heating gas of hydrogen was at thesame time introduced into the vessel 10. The pressure of the hydrogengas in the vessel 10 was increased gradually as the temperature of thearticles 22 increased. No external or auxiliary heating was employed,and the articles 22 were heated by the glow discharge only.

In other embodiments (not shown), an external heater attached to thevessel may be employed, or a combination of external heating andelectrical glow discharge heating may be employed. Direct current (dc)discharge, pulsed dc discharge or alternating current (ac) discharge maybe used.

After the articles 22 were heated to the prescribed temperature, a gasmixture of hydrogen (98.5%) and methane (1.5%) was introduced into thevessel 10 and the plasma treatment started. Treatment temperatures from400° C. to 600° C. were employed for a treatment time of 10 hours. Theworking pressure in the treatment step was 500 Pa (5.0 mbar) for all theExamples.

During the plasma heat treatment, the methane is ionised, activated anddissociated to produce carbon ions and activated carbon atoms andneutral molecules, which then diffuse into the surface of the discforming a carbon diffusion layer. When the plasma treatment is carriedout at a relatively low temperature ranging from 300 to 550° C., thecarbon atoms mainly reside in the cobalt lattices, forming asupersaturated solid solution with a possible nanocrystalline structuredue to the relatively low temperatures employed in the treatment. Theresultant layer has a high hardness, good fatigue strength and excellentwear and corrosion resistance (see below). When the plasma treatment iscarried out at a relatively high temperature ranging from 600 to 700°C., the carbon atoms partially reside in the cobalt lattices forming asupersaturated solid solution and partially combined with carbon formingchromium carbides. The resultant layer has a high hardness, fatiguestrength and excellent wear resistance.

After the completion of the plasma treatment, the glow discharge wasturned off and the articles 22 were allowed to cool in the vessel 10 inthe treatment atmosphere down to room temperature before they wereremoved from the vessel.

The articles 22 were then subjected to X-ray diffraction analysis forphase identification, glow discharge spectrometry (GDS) analysis forchemical composition determination, surface hardness measurements,metallography analysis of the cross section, electrochemical corrosiontests and wear tests.

A typical micrograph showing the cross-section of the carburised sampleis given in FIG. 2. The sample was electrolytically etched in a 10%H₂SO₄ water solution. It can be seen that the carburised specimen ischaracterised by an “un-etched layer” on the surface followed by acarbon diffusion layer (“case”) beneath and the heavily etched substrate(“core”). The un-etched layer is dense, no details can be revealed evenunder high resolution FEG-SEM. The thickness of the total carburisedcase depth increases with increasing carburising temperature. Inconjunction with the chemical composition analysis, the case depth wasdetermined to be 3.1, 9.3 and 20.2 μm respectively for the A-sampletreated at 400° C., 500° C. and 600° C. for 10 hours.

FIG. 3 shows the XRD patterns for the untreated and carburisedCo-27.6Cr-5.5Mo-0.06C alloy (Sample A). As can be seen, the untreatedalloy consists of a mixture of f.c.c. structured γ-Co and h.c.p.structured ε-Co. It can be deduced from the height of the XRD peaks thatthe untreated alloy contains mainly γ with a small amount of ε. Plasmacarburising has changed the phase constituent in the alloy surface. Asshown in FIG. 3, only two main diffraction peaks at lower angles andsome minor peaks at higher angles are detected. These peaks could not bematched to either γ-Co, ε-Co or any other phases given in the existingPowder Diffraction Database. However, they exhibit the samecharacteristics as those generated by the S-phase in plasma nitrided orcarburised stainless steel (Y. Sun X. Li and T. Bell SurfaceEngineering, 1999, Vol 15, No. 1, pp 49-54). It thus follows thatS-phase Was indeed produced in the plasma carburised Co—Cr—Mo alloysurface. The two main S-phase peaks, indicated as S(111) and S(200),correspond to the γ(111) and γ(200) of the substrate but are at lowerangles. It is thought that the peak shift is caused by the solution ofcarbon which expands the f.c.c lattice structure of the substrate.

The surface mechanical properties of the plasma carburised samples wereassessed using a Mitutoyo MVK-H tester under various loads, ranging from0.025 kg to 1 kg. FIG. 4 shows micro hardness measured on the sample Asurface with various load. It can be seen that the hardness on theuntreated surface (A000) is fairly stable under a testing load of above0.05 kg, with an average value of HV 486. After carburising, the surfacehardness of the Co—Cr—Mo alloy was increased under all testing loads.Under indentation loads below. 0.1 kg, surface hardness values of morethan 1100 HV were obtained for the 500° C. and 600° C. treated samples(A500 and A600 respectively). The surface hardness values for the 400°C. treated sample (A400) were not as high as those for the 500° C. and600° C. treated samples, but they are still much higher than those forthe untreated alloy. As the testing load increased, all the measuredhardness values decreased, showing a hardness gradient with testing loadand this with the indentation depth. Such a diffusion type of hardnessdistribution is essential in ensuring optimum performance of surfacetreated system, since a sudden structural, compositional, and propertychange at the layer/core interface may lead to catastrophic interfacialfailure of the layer during service. It was also found that the edges ofthe Vickers indentation impressions on the carburised samples were sharpand clear. No cracks were observed around or inside the indentationimpression, and there was no evidence of interfacial failure between thecarburised layer and the substrate even at a higher load of 1 kg. Theseobservations suggest that the carburised layer possesses good ductility,high toughness and high load bearing capacity.

Plasma carburised and untreated test pieces were subjected to pin (WCball)-on-disc wear tests sliding against 8 mm WC balls at a speed of0.03 m/s with a initial maximum Hertzian contact stress of 1500 MP. Wearrate, in terms of volume loss per meter sliding distance per Newton load(mm³m⁻¹N⁻¹), was calculated and is shown in FIG. 5. As can be seen fromFIG. 5, the wear rate of all the carburised test pieces was dramaticallyreduced by more than one order of magnitude as compared with theuntreated test pieces.

Electrochemical potentiondynamic sweep corrosion tests were conducted atroom temperature in a flat cell with Ringer's solution which contained 9WI sodium chloride, 0.42 g/l potassium chloride, 0.48 g/l calciumchloride and 0.2 g/l sodium bicarbonate in distilled water. Thepolarisation curves are shown in FIG. 6. As compared with the untreatedtest piece, the corrosion potentials of all plasma carburised testpieces were moved to more passive values, indicating improved corrosionresistance. The current densities of both the untreated and the A400 andA500 test pieces are practically the same. The A600 test piece showed ahigher current density scan potential was over −0.2V.

The applicability of the present invention is demonstrated in Table 3.Treatment was carried out at 500° C. for 10 hrs in a gas mixture ofmethane and hydrogen. Microhardness was measured on the treated samplewith a Vickers indenter at a load of 0.1 kgf, indicated as HV0.1. As canbe seen, all five types of Co—Cr based alloys in wrought, cast or PM/HIPform can be effectively surface hardened by applying the method of thepresent invention.

TABLE 3 Surface hardness of samples A to F. Sample Surface HardnessNumber Condition HV0.1 A Wrought 1200 B Wrought 1150 C Cast MMT) 1200 DCast (DEP) 1000 E Cast (DRILL) 1240 F PM/HIP 960

In a modification of the above dc plasma method not shown), an advancedactive screen plasma technology can be used to treat articles made ofCo—Cr based alloys with improved surface quality. The articles to beplasma treated are placed inside a metal screen which is connected tothe cathodic potential. The worktable and the articles to be treated areplaced in a floating potential or subject to a relatively lower biasvoltage (e.g. −100˜200V). As an example, a casting material (material Din Table 3) has been plasma treated in a active screen plasma unit andthe surface hardness increased from <4001HV0.1 (untreated material) to˜10501-HV0.1 (active screen plasma treated).

1. A method of improving surface hardness wear resistance or fatiguestrength of a cobalt-chromium based alloy medical implant without lossin corrosion resistance, the method comprising: plasma treating theimplant at a temperature in the range of from 300° C. to 550° C. and ata pressure of from 100 Pa to 1500 Pa for 1 hour to 50 hours in anatmosphere comprising at least one carbon-containing gas, whereby tointroduce carbon into a surface region of said implant, to produce asupersaturated solid solution of carbon in cobalt.
 2. The method ofclaim 1, wherein the medical implant is a joint or knee prosthesis. D.The method of claim 1, wherein the plasma treating is carried out at atemperature in the range of from 350° C. to 550° C.
 4. The method ofclaim 3, wherein the plasma treating is carried out at a temperature inthe range of from 400° C. to 500° C.
 5. The method of claim 1, whereinthe treatment pressure is in the range of from 400 Pa to 600 Pa.
 6. Themethod of claim 1, wherein the duration of said treatment is in therange of from 5 hours to 30 hours.
 7. The method of claim 1, wherein thecarbon-containing gas is selected from a hydrocarbon, carbon dioxide,and carbon monoxide.
 8. The method of claim 1, wherein the plasmatreatment is carried out in the presence of at least one other gasselected from hydrogen, helium, argon or other noble gas.
 9. The methodof claim 1, wherein the plasma treatment is carried out in the presenceof at least one additional gas to be incorporated into the surfaceregion of said implant in addition to the carbon.
 10. The method ofclaim 9, wherein the additional gas is a nitrogen containing gas. 11.The method of claim 9, wherein said additional gas constitutes from 0.5%to 10% by volume of the total atmosphere.
 12. The method of claim 8,wherein the at least one other gas is hydrogen or a mixture of hydrogenand argon, and the carbon-containing gas is methane.
 13. The method ofclaim 1, wherein the carbon-containing gas constitutes from 0.5% to 20%by volume of the total atmosphere.
 14. The method of claim 1, whereinsaid plasma treatment is effected in the absence of oxygen.
 15. Themethod of claim 1, further comprising a medical implant cleaning stepprior to the plasma treatment to remove oxide scale.
 16. The method ofclaim 15, wherein cleaning is effected by sputter cleaning.
 17. Themethod of claim 15, wherein said cleaning step is effected at or belowthe subsequent plasma treatment temperature in an atmosphere of one ormore gases selected from hydrogen, helium, argon or other noble gas. 18.The method of claim 1, wherein the medical implant is cooled after theplasma treatment.
 19. The method of claim 18, wherein the rate ofcooling is from 0.1° C./min up to 1000° C./min.
 20. The method of claim18, wherein the cooling is achieved by relatively slow cooling in theplasma treating atmosphere or by relatively fast cooling by quenching ina liquid.
 21. The method of claim 1, wherein at least one of apassivation step and a polishing step is effected after completion ofthe plasma treatment.
 22. The method of claim 1, wherein thecobalt-chromium based alloy includes one or more other alloyingingredients selected from molybdenum, nickel, tungsten, titanium andcarbon.
 23. A surface-hardened cobalt-chromium based alloy medicalimplant producible by the method of claim 1, said implant characterisedby having a surface region comprising a supersaturated solid solution ofcarbon in cobalt.
 24. The medical implant of claim 23, wherein saidsurface region has a thickness in the range of from 3 μm to 50 μm. 25.The medical implant of claim 23, wherein the cobalt-chromium based alloyincludes one or more other alloying ingredients selected frommolybdenum, nickel, tungsten, titanium and carbon.
 26. The medicalimplant of claim 25, wherein carbon is present as an alloying ingredientin an amount of from 0.04 wt % to 1.6 wt %.
 27. The medical implant ofclaim 26, wherein carbon is present as an alloying ingredient in therange of from 0.04 wt % and 0.4 wt %.
 28. The medical implant of claim23, wherein the implant includes at least one of a conventional hip orknee joint prostheses, a metal-on-metal advanced bone conservationprostheses, or a dental implant.